Ion mobility spectrometer clear-down

ABSTRACT

Method and systems for managing clear-down are provided. The method can include generating a clear-down trigger associated with an ion mobility spectrometer and operating the ion mobility spectrometer in fast clear-down mode in response to the clear-down trigger. Methods and systems can further provide that where the ion mobility spectrometer operates in fast-switching mode, the ion mobility spectrometer alternating a plurality of times between operation according to a positive ion mode and operation according to a negative ion mode, and further operating according to the positive ion mode for less than about 1 second before switching to the operation according to the negative ion mode, and operating according to the negative ion mode for less than about 1 second before switching to the operation according to the positive ion mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/881,817, filed Jul. 5, 2013, now allowed, which is a § 371 U.S.National Entry of PCT/US2011/058046, filed Oct. 27, 2011, which claimsthe benefit of U.S. Provisional Patent Application Ser. No. 61/407,335,filed Oct. 27, 2010, U.S. Provisional Patent Application Ser. No.61/407,327, filed Oct. 27, 2010, and U.S. Provisional Patent ApplicationSer. No., filed 61/407,342, Oct. 27, 2010, each of which is incorporatedby reference in its entirety.

FIELD OF THE INVENTION

This application is directed to the maintenance of ion mobilityspectrometers.

BACKGROUND OF THE INVENTION

Ion mobility spectrometry is a method used to identify the compositionof a sample of ions using ion mobility. Ion mobility spectrometers canbe employed at security checkpoints, such as airports, to assist in thedetection of explosives and narcotics. When used at airports, forexample, residue from luggage can be transferred to a swab, which can bemanipulated so that molecules and/or atoms associated with the residuepass into an ionization region within the ion mobility spectrometer. Inthe ionization region, the molecules and atoms associated with theresidue can be ionized. Both positive and negative ions can form in theionization region. An electric field at grids spaced between theionization region and a drift region can be pulsed to allow ions to passfrom the ionization region into the drift region. The ions in the driftregion can be further subject to a force as a result of an electricfield maintained in the drift region. Once in the drift region, the ionscan separate based upon the ions' respective ion mobility. In this way,a time-of-flight measurement of the ions in the drift region (which canbe measured as a change in current magnitude on a collector plate at oneend of the drift region), can provide an identifying peak in a measuredcurrent magnitude, and which can be associated with a particular ion.The plot of current magnitude at the collector as a function of time isreferred to as a plasmagram.

SUMMARY

In one aspect, embodiments provide a method of managing clear-down. Themethod can include operating the ion mobility spectrometer infast-switching mode in response to a clear-down trigger. Embodiments canfurther provide that the ion mobility spectrometer in fast-switchingmode alternates a plurality of times between operation according to apositive ion mode and operation according to a negative ion mode, andfurther operates in positive ion mode for less than about 1 secondbefore switching to negative ion mode, and operates in negative ion modefor less than about 1 second before switching to positive ion mode.

In another aspect, embodiments can provide an ion mobility spectrometerthat can include a repelling grid, a gating grid, an ionization region,a drift region, and a collector. Embodiments can further provide thatthe ionization region, the repelling grid, the gating grid, and thedrift region are configured to switch from positive ion mode to negativeion mode in fast-switching mode. Further still, embodiments can providethat the ionization region, the repelling grid, the gating grid, and thedrift region are configured to switch from negative ion mode to positiveion mode in fast-switching mode, and wherein the ion mobilityspectrometer is configured to be responsive to a clear-down trigger sothe ion mobility spectrometer operates in fast-switching mode. In thisaspect, an ion mobility spectrometer in fast-switching mode can operatein positive ion mode for less than about 1 second before switching tonegative ion mode, and can operate in negative ion mode for less thanabout 1 second before switching to positive ion mode.

In a further aspect, embodiments can provide a computer-readable mediumcomprising instructions stored thereon, wherein the instructions cause aprocessor to perform a method of managing fast clear-down. The methodcan include operating an ion mobility spectrometer in fast-switchingmode in response to a clear-down trigger. Embodiments consistent withthe present disclosure can further provide that the ion mobilityspectrometer in fast-switching mode alternates a plurality of timesbetween operation in positive ion mode and operation in negative ionmode, and further operate in positive ion mode for less than about 1second before switching to the negative ion mode, and operate innegative ion mode for less than about 1 second before switching topositive ion mode.

Additional features and embodiments of the invention will be set forthin part in the description which follows, and in part will be obviousfrom the description, or may be learned by practice of the invention. Itis to be understood that both the foregoing general description and thefollowing detailed description are exemplary and explanatory only andare not restrictive of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate exemplary embodiments andtogether with the description, serve to explain the principles of thedisclosure.

FIG. 1 is a cross-sectional view of an ionization region and a driftregion of an ion mobility spectrometer consistent with an embodiment.

FIG. 2 depicts an example two-dimensional plasmagram associated with afirst set of segment measurements of a blank swab in positive ion mode.

FIG. 3 depicts an example two-dimensional plasmagram associated with asecond set of segment measurements of a blank swab in negative ion modein which reactant has not been introduced into the ionization region.

FIG. 4 depicts an example two-dimensional plasmagram associated with athird set of segment measurements of a blank swab in negative ion modein which reactant has been introduced into the ionization region.

FIG. 5 depicts an example three-dimensional plasmagram associated with afirst set of segment measurements in positive ion mode, negative ionmode with no reactant, and negative ion mode with reactant.

FIG. 6 depicts a data processing system consistent with an embodiment.

FIG. 7 is a flowchart depicting a method of clearing-down an ionmobility spectrometer consistent with an embodiment.

FIG. 8 is a flowchart depicting a method of clearing-down an ionmobility spectrometer consistent with an embodiment.

FIG. 9 is a flowchart depicting a method of clearing-down an ionmobility spectrometer consistent with an embodiment.

FIG. 10 is a flowchart depicting a method of clearing-down an ionmobility spectrometer consistent with an embodiment.

FIG. 11 depicts exemplary voltages as a function of time on therepelling grid, the gating grid, and the fixed grid of the ion mobilityspectrometer of FIG. 1 during a change from positive ion mode tonegative ion mode.

FIG. 12 is a cross sectional view of a guard grid and a collectorportion of an ion mobility spectrometer consistent with an embodiment.

FIG. 13 is a schematic of a circuit diagram consistent with anembodiment.

FIG. 14 depicts a timing trace of voltage switching consistent with anembodiment.

FIGS. 15 and 16 depict a further switch consistent with an embodiment ofthe circuit of FIG. 13.

FIG. 17 depicts a plasmagram before and after it has been normalized.

FIG. 18 schematically depicts steps associated with the normalization ofthe plasmagram of FIG. 17 consistent with an embodiment.

DETAILED DESCRIPTION

Reference will now be made in detail to the disclosed embodiments of thedisclosure, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

A portion of an ion mobility spectrometer 100 consistent with anembodiment of the current disclosure is depicted in FIG. 1. Moleculesand/or atoms associated with a sample being tested can enter through aninlet 110 (depicted with arrow 105). Sample molecules and/or atoms thenpass a repelling grid 125 into an ionization region 140. The repellinggrid 125 can comprise inert metal (e.g., gold-plated nickel), and canhave a grid spacing of about 0.1 mm. The ionization region 140 caninclude a region with an ionization source 130. The ionization source130 can comprise a material such as Nickel-63. Alternatively, ions canbe created in ionization region 140 as a result of corona dischargeionization, photoionization, electrospray ionization, matrix assistedlaser desorption ionization (MALDI), or the like.

The ion mobility spectrometer 100 can operate in positive ion mode andnegative ion mode. In these modes, certain components of the ionmobility spectrometer 100 can exhibit voltages in order to create anelectric field along the length of the ion mobility spectrometer 100.When the ion mobility spectrometer 100 is operating in positive ionmode, for example, the repelling grid 125 can exhibit a relatively highpositive voltage. As described further below, when operating in positiveion mode, other components of ion mobility spectrometer 100 locatedtowards the opposite end of the ionization region 140 and across thedrift region 160 will exhibit lower voltages. This configuration willcreate an electric field in the ionization region 140, for example, thatdirects positive ions away from the inlet 110. In an embodiment, themagnitude of the positive voltage on the repelling grid 125 can be about2100 V. The range of magnitudes of the positive voltage on the repellinggrid 125 can be 1000V to 5000V. Example values can be higher or lowerdepending upon the physical dimensions of the system. Both a fixed grid135 and a gating grid 145 are located between the ionization region 140and the drift region 165. As discussed above, and in positive ion mode,the fixed grid 135 can exhibit a voltage that is less than the positivevoltage on the repelling grid 125 such that there is a potentialgradient (i.e., an electric field) oriented across the ionization region140. Other components can also be present between the repelling grid 125and the fixed grid 135 in the ionization region 140 in support of anelectric field in the ionization region 140. In an embodiment, when thevoltage on the repelling grid 125 is approximately 2100 V as describedabove, the fixed grid 135 can exhibit a positive voltage that isapproximately 1810 V. The voltage on the fixed grid 135 can be chosen sothe potential gradient near the fixed grid 135 in the ionization region140 and near the fixed grid 135 in the drift region 165 will provide aforce on ions that will direct the ions from the ionization region 140to the drift region 165 when the gating grid 145 is “open” (as isdescribed further below). By way of example only, and withoutlimitation, a configuration that allows for a uniform electric fieldacross the barrier between the ionization region 140 and the driftregion 165 is a configuration that can provide a uniform force on an ionto direct ions (of one polarity) from the ionization region 140 to thedrift region 165. According to the above embodiment, an electric fieldacross the ionization region 140 can have a magnitude that ranges from50 V/cm to 500 V/cm. The electric field in the ionization region 140does not need to be uniform throughout the ionization region 140.However, the electric field in the drift region 165 can be generallyuniform. For example, where the drift region 165 is approximately 6.9cm, and the electric field across the drift region 165 also has amagnitude of 250 V/cm, the voltage on a guard grid 175 at one end ofdrift region 165 can be approximately 90 V. In other embodiments, therange of values for an electric field in the drift region 165 can be 200V/cm to 300 V/cm.

Adjacent to the fixed grid 135 is the gating grid 145, where the gatinggrid 145 can be positioned so the fixed grid 135 is between therepelling grid 125 and the gating grid 145. The gating grid 145 can beapproximately 0.75 mm from the fixed grid 135. A shutter structureconsistent with the combination of the fixed grid 135 and the gatinggrid 145 is referred to as a Bradbury-Nielsen gate. (Without limitation,another shutter structure consistent with the present disclosure is aTyndall's gate.) The combination of the fixed grid 135 and the gatinggrid 145 can comprise two sets of parallel wires (which can be twoetched foils), where the spacing between the wires of the respectivegrids can be about 0.8 mm. The parallel wires on the grids can beoriented in the same direction, but can be spaced so that, when viewedfrom a direction that is perpendicular to the plane of the grids, thewires are interleaved. There can also be an insulating foil of thicknessabout 0.75 mm between the grids. The fixed grid 135 and the gating grid145 can comprise Invar or other materials. In positive ion mode, thegating grid 145 can be kept at a higher voltage than the fixed grid 135to create a barrier along the potential gradient between the ionizationregion 140 and the drift region 165. When the gating grid 145 is at ahigher potential than the fixed grid 135, the gating grid 145 isreferred to as “closed.” The difference in voltage between the gatinggrid 145 and the fixed grid 135, when the gating grid 145 is closed, canbe about 20 V. The voltage of the gating grid 145 can have a magnitudeof about 1830 V in positive ion mode. Such a magnitude can have theeffect of introducing an electric field that interferes with the passageof positive ions from the ionization region 140 through the drift region165 to a collector 170 (described further below).

After molecules and/or atoms have entered the ionization region 140 andpositive ions form, the repelling grid 125 can be maintained at a highvoltage as described above and the gating grid 145 can remain closed forapproximately 20 milliseconds. After this time period elapses, anegative voltage pulse can be applied to the gating grid 145 to open thegating grid 145 and allow positive ions to move from the ionizationregion 140 to the drift region 165 so the positive ions may traveltoward the collector 170. In an embodiment, when the gating grid 145 isapproximately 20 V higher than the fixed grid 135 when closed, thenegative voltage pulse to the gating grid 145 can have an amplitude ofapproximately 25 V to open the gating grid 145. In an embodiment, thenegative voltage pulse applied to the gating grid 145 to open the gatinggrid 145 can have an amplitude so the potential gradient at the boundarybetween the ionization region 140 and the drift region 165 directspositive ions from the ionization region 140 to the drift region 165 sopositive ions can arrive at the collector 170. A time period permittedfor the ions to move from the ionization region 140 to the drift region165 (when the gating grid 145 is open) can be about 200 microseconds.The gating grid 145 can be open for about 200-300 microseconds, but canbe open for as short as about 50 microseconds and open for as long asabout 1000 microseconds. Opening the shutter structure (such as bypulsing the voltage on the gating grid 145) for this duration, and thenclosing the shutter structure can allow positive ions to move into thedrift region 165 so the positive ions can arrive at the collector 170.In the drift region 165, an electric field can provide a force on thepositive ions to direct the positive ions through the drift region 165towards the guard grid 175 and the collector 170. The collector 170 canbe any suitable structure for detecting pulses of current associatedwith moving ions, such as a Faraday plate. As the positive ions movethrough the drift region 165 towards the collector 170, the positiveions can move through a drift gas. In an embodiment, the drift gas canmove in the opposite direction to the flow of the positive ions, wherethe flow of positive ions is towards the collector 170. The drift gascan enter the drift region 165 from a drift flow 180 (indicated by arrow185) and exit the ion mobility spectrometer 100 through an exhaust flow150 (indicated by arrow 155). The drift gas in the drift region 165 canbe dry air, although other gases such as nitrogen or helium can be used.As the ions move through the drift region 165 toward the collector 170,the various species of ions can separate as a function of theirmobility. The drift time of the ions across the drift region 165 canvary, depending on their atomic and molecular characteristics and thetemperature and pressure of the drift gas. For a drift region that isapproximately 6.9 cm in length and at normal atmospheric pressure andtemperature, the drift time can be in the range of 5 milliseconds to 20milliseconds. Furthermore, the time period during which data is acquiredfrom the collector 170 associated with one scan can range from about 2milliseconds to about 40 milliseconds. In an embodiment, one scan canrepresent a 25 millisecond time period.

Accordingly, electric current values can be measured at regular timeintervals at the collector 170, corresponding to time-of-flightsignatures of the ionic species that can make up the positive ionspresent in the drift region 165. As discussed above, in an embodiment,the drift gas can flow in the opposite direction from the movement ofthe positive ions being measured at the collector 170 in positive ionmode. Such a drift gas flow can be used to keep the drift gas pure, buta flow is not required for operation of the ion mobility spectrometer100. Other methods and systems for maintaining drift gas purity caninclude placing sorbent material within the drift region 165.

In an embodiment, as described above, the voltage difference between thegating grid 145 and the guard grid 175 can be approximately 1720 V andthe distance between the gating grid 145 and the guard grid 175 can be6.9 cm. The magnitude of the voltage of the guard grid 175 can beapproximately 90 V.

Drift rings 160 can be employed in drift region 165. In an embodiment,the drift rings 160 can be flat metal rings, spaced at regular intervalsbetween the gating grid 145 and the guard grid 175 and can be biased atequal voltage steps to improve uniformity of the potential gradient(that is, the uniformity of the electric field) within the drift region165.

Operation of the ion mobility spectrometer 100 in negative ion mode issimilar, in principle, to its operation in positive ion mode. Therelative voltages on the repelling grid 125, the fixed grid 135, thegating grid 145, and the guard grid 175, however, are inverted.Specifically, the repelling grid 125 can be more negative than the fixedgrid 135, which can be more negative than the guard grid 175. In anembodiment of the ion mobility spectrometer 100 operating in negativeion mode, the magnitude of the voltages associated with the repellinggrid 125, the fixed grid 135, the gating grid 145, and the guard grid175 can be approximately similar in magnitude but with opposite polarityto those recited above in positive mode. Specifically, the repellinggrid 125 can be approximately −2100 V, the fixed grid 135 can beapproximately −1810 V, the guard grid 175 can be approximately −90 V,and the gating grid 145 can be approximately −1830 V when closed, andpulsed to approximately −1805 V when open. The voltage across the driftrings 160 can also be inverted from the circumstance described inpositive ion mode to form a uniform potential gradient through the driftregion 165. In this way, the potential gradient in negative ion mode isinverted from the potential gradient described above in connection withpositive ion mode, thereby inverting the direction of the electric fieldacross the ionization region 140 and the drift region 165 of the ionmobility spectrometer 100.

As described above, the drift region 165 can have an electric fieldapplied along its length, and the slope of the potential field as afunction of distance (i.e., the direction of the electric fieldassociated with the potential gradient) can be positive or negativedepending on the charge of the ions. Ions of a similar polarity can movefrom the ionization region 140 into the drift region 165 by the openingand closing of the gating grid 145. The time period of a scan of acollection of ions in the drift region 165 is the time period betweenwhen the gating grid 145 opens to admit ions into the drift region 165from the ionization region 140, and the subsequent opening of the gatinggrid 145 to admit additional ions into the drift region 165 from theionization region 140. The interval between subsequent voltage pulsesapplied to gating grid 145 so that it opens (i.e., negative voltagepulses for operation in positive ion mode and positive voltage pulsesfor operation in negative ion mode) is referred to as the “scan period.”Current measurements that are acquired from the collector 170 fromseveral subsequent scans can be co-added together to improvesignal-to-noise of the mobility spectrum reflected in the scans. Thiscollection of data is referred to as a “segment.” Data associated withone segment can be acquired in less than a second (i.e., data associatedwith one segment can be acquired by co-adding approximately 40 scans orless, where the scans have a duration of approximately 25 milliseconds).A series of sequential segments, with characteristic ion peak patterns,can be obtained and can be displayed either as a series of individualsegments versus desorption time in seconds (a three-dimensionalplasmagram) or as an average of all segments obtained during theanalysis (a two-dimensional plasmagram). The desorption time is the timeassociated with the desorption of molecules and atoms from the swab,such as through the application of heat. The desorption of the moleculesand atoms from the swab through the application of heat, for example,can make the molecules and atoms available to pass through the inlet 110and into the ionization region 140.

As described above, in positive ion mode, the gating grid 145 can bekept at a higher voltage than the fixed grid 135 to create a barrieralong the potential gradient between the ionization region 140 and thedrift region 165. When the gating grid 145 is at a higher potential thanthe fixed grid 135, the gating grid 145 is referred to as “closed.”Further, as described above, the difference in voltage between thegating grid 145 and the fixed grid 135, when the gating grid 145 isclosed, can be about 20 V. Such a magnitude can have the effect ofsupporting an electric field that interferes with the passage ofpositive ions from the ionization region 140 through the drift region165 to the collector 170. Moreover, a negative voltage pulse can beapplied to the gating grid 145 to open the gating grid 145 and allowpositive ions to move from the ionization region 140 to the drift region165 so the positive ions may travel toward the collector 370. In anembodiment, when the gating grid 145 is approximately 20 V higher thanthe fixed grid 135 when closed, the negative voltage pulse to the gatinggrid 145 can have an amplitude of approximately 25 V to open the gatinggrid 145. In a further embodiment, a positive voltage pulse ofapproximately 25 V can be applied to the fixed grid 135, while thegating grid 145 is left unchanged in order to “open” the shutterstructure associated with the combination of the fixed grid 335 and thegating grid 345 in positive ion mode. That is, in a further embodiment,and rather than applying a negative voltage pulse to the gating grid 145while the fixed grid 135 is left unchanged, a positive voltage pulse canbe applied to the fixed grid 135 while the gating grid 145 is leftunchanged. Further still, in further embodiments, a positive voltagepulse of approximately N volts can be applied to the fixed grid 135 anda negative voltage pulse of approximately 25−N volts can be applied tothe gating grid 145 in order to “open” the shutter structure associatedwith the combination of the fixed grid 135 and the gating grid 145 inpositive ion mode.

Further still, and as described above, in negative ion mode, the gatinggrid 145 can be kept at a lower voltage than the fixed grid 135 tocreate a barrier along the potential gradient between the ionizationregion 140 and the drift region 165. When the gating grid 145 is at alower potential than the fixed grid 135, the gating grid 145 is referredto as “closed.” Further, as described above, the difference in voltagebetween the gating grid 145 and the fixed grid 135, when the gating grid145 is closed, can be about 20 V. Again, such a magnitude can have theeffect of supporting an electric field that interferes with the passageof negative ions from the ionization region 140 through the drift region165 to the collector 170. Further still, a positive voltage pulse can beapplied to the gating grid 145 to open the gating grid 145 and allownegative ions to move from the ionization region 140 to the drift region165 so the negative ions may travel toward the collector 170. In anembodiment, when the gating grid 145 is approximately 20 V lower thanthe fixed grid 135 when closed, the positive voltage pulse to the gatinggrid 145 can have an amplitude of approximately 25 V to open the gatinggrid 145. In a further embodiment, a negative voltage pulse ofapproximately 25 V can be applied to the fixed grid 135, while thegating grid 145 is left unchanged in order to “open” the shutterstructure associated with the combination of the fixed grid 135 and thegating grid 145 in negative ion mode. That is, in a further embodiment,and rather than applying a positive voltage pulse to the gating grid 145while the fixed grid 135 is left unchanged, a negative voltage pulse canbe applied to the fixed grid 135 while the gating grid 145 is leftunchanged. In further embodiments, a negative voltage pulse ofapproximately N volts can be applied to the fixed grid 135 and apositive voltage pulse of approximately 25−N volts can be applied to thegating grid 145 in order to “open” the shutter structure associated withthe combination of the fixed grid 135 and the gating grid 145 innegative ion mode.

FIGS. 2-4 depict example plasmagrams associated with the current valuesmeasured at the collector 170. FIG. 2 is an example two-dimensionalplasmagram 201 associated with an ion mobility spectrometer, such as ionmobility spectrometer 100, operating in positive ion mode. The abscissaof the two-dimensional plasmagram 201, the drift time, is the amount oftime after the gating grid 145 opens to allow ions into the drift region165 so that the ions can arrive at the collector 170. That is, when theion mobility spectrometer 100 is operating in positive ion mode, thezero of the drift time abscissa corresponds to the negative voltagepulse that opens the gating grid 145. The ordinate of thetwo-dimensional plasmagram 201 is the current signal acquired at thecollector 170 as a function of the drift time. The units associated withthe ordinate of the two-dimensional plasmagram 201 can be arbitrary, asthe measured current at the collector 170 can be a function of a numberof design parameters associated with the construction and operation ofthe ion mobility spectrometer 100. As described above, a plurality ofscans can be co-added together to form a segment. In the two-dimensionalplasmagram 201 depicted in FIG. 2, each scan in the plurality of scansthat make up a segment occurs for at least 21.9 milliseconds, and thesegments numbered 1 through 4 (all in positive ion mode and occurringover 1.41 seconds of desorption time) are averaged together. Two peaksare visible in FIG. 2: a nicotinamide peak 202 (labeled in FIG. 2 as“Cal(+)”) and a hydronomium peak 203.

FIG. 3 is an example two-dimensional plasmagram 301 associated with theion mobility spectrometer 100 operating in negative ion mode. As withFIG. 2, the abscissa of the two-dimensional plasmagram 301, the drifttime, is the amount of time after the gating grid 145 opens to allowions into the drift region 165. Note, however, that when the ionmobility spectrometer 100 is operating in negative ion mode, the zero ofthe drift time abscissa corresponds to the positive voltage pulse thatopens the gating grid 145. The ordinate of the two-dimensionalplasmagram 301 is the current signal acquired at the collector 170 as afunction of the drift time according to the same units associated withFIG. 2. Again, as described above, a plurality of scans can be co-addedtogether to form a segment, and again, as described in connection withFIG. 2, each scan in the plurality of scans that make up a segment inthe two-dimensional plasmagram 301, occurs for at least 21.9milliseconds, and the segments numbered 5 through 8 (all in negative ionmode and occurring between 1.41 seconds and 3.61 seconds of desorptiontime) are averaged together. Several peaks are visible in FIG. 3,including an oxygen peak 302 and a nitrobenzonitrile peak 303 (labeledin FIG. 3 as “Cal(−)”).

The sequence of two-dimensional plasmagrams 201 and 301 reflect acircumstance where the ion mobility spectrometer 100 has operated inpositive ion mode for approximately 1.41 seconds (acquiring the data forsegments 1-4), and then switched to operation in negative ion mode andstarting negative ion mode scans (at approximately 1.41 seconds intodesorption time). Thus, the data reflected in FIGS. 2 and 3 indicatethat the ion mobility spectrometer 100 has been operating in bothpositive ion mode and negative ion mode, and that both dopants(nitrobenzonitrile and nicotinamide) and water are present. FIGS. 2 and3 are part of an explosives-swab mode analysis. The two-dimensionalplasmagram 401 depicted in FIG. 4 is also part of the explosives-swabmode analysis, and corresponds to an averaging of segments 9-19 acquiredduring desorption time 3.61 seconds to 7.83 seconds in negative ionmode. One difference between the circumstance resulting in thetwo-dimensional plasmagram 301 and the two-dimensional plasmagram 401 isthat plasmagram 401 is associated with the presence of the reactanthexachlooroethane in the ion mobility spectrometer 100. In thetwo-dimensional plasmagram 401 depicted in FIG. 4, a nitrobenzonitrilepeak 404 is less prominent than the oxygen peak 403 and a reactant peak402 and a reactant peak 405.

FIG. 5 depicts an exemplary three-dimensional plasmagram 501. In fact,the three-dimensional plasmagram 501 depicts segments 1-19 (acquiredduring desorption time 0 seconds to 7.83 seconds) corresponding to theexplosives-swab mode analysis of FIGS. 2-4. The three-dimensional viewdepicted in FIG. 5 reflects the pattern depicted in FIGS. 2-4: positiveion mode depicted near the abscissa (segments 1-4) corresponding to thetwo-dimensional plasmagram 201 in FIG. 2; negative ion mode with noreactant (segments 5-8) corresponding to the two-dimensional plasmagram301 in FIG. 3; and negative ion mode with reactant added (segments 9-19)corresponding to the two-dimensional plasmagram 401 in FIG. 4.

Consistent with an embodiment, the ion mobility spectrometer 100includes a data processing system 600. FIG. 6 is a schematic diagram ofthe data processing system 600. The data processing system 600 caninclude a processor 601, a memory module 604, a collector interface 603,a storage 602, a user input interface 605, a display 606, a gatinginterface 607, and a mode polarity manager 608. The data processingsystem 600 can include additional, fewer, and/or different componentsthan those listed above. The type and number of listed devices areexemplary only and not intended to be limiting.

The processor 601 can be a central processing unit (“CPU”) and/or agraphic processing unit (“GPU”). The processor 601 can execute sequencesof computer program instructions to perform various processes that willbe explained in greater detail below. The memory module 604 can include,among other things, a random access memory (“RAM”) and a read-onlymemory (“ROM”). The computer program instructions can be accessed andread from the ROM, the storage 602 (such as a software 610), or anyother suitable memory location, and loaded into the RAM for execution bythe processor 601. Although the software is depicted as being stored onstorage 602, e.g., a hard drive, the instructions comprising thesoftware may be stored in a wide variety of tangible storage media. Itis the intention of this disclosure to encompass such variations.Depending on the type of data processing system 600 being used, theprocessor 601 can include one or more processors included on printedcircuit boards, and/or microprocessor chips.

Collector interface 603 can be configured to receive signals from thecollector 170 such that processor 601, for example, may store datarepresenting the signals output by the collector in the storage 602.

The storage 602 can include any type of storage suitable for storinginformation. For example, the storage 602 can include one or more harddisk devices, optical disk devices, or any other storage devices thatcan retain the data. In an embodiment, the storage 602 can store datarelated to the data processing process, such as the scan data receivedfrom the collector 170, and any intermediate data created during thedata processing process. The storage 602 can also include analysis andorganization tools for analyzing and organizing the informationcontained therein, such as a data library 612 that can include dataassociated with plasmagram peak positions, peak amplitudes, peak widths,and/or reduced ion mobility values. In addition, the gating interface607, via the hardware included in the data processing system can beconfigured to provide a signal, such as a pulse, to open the gating grid145.

A user may implement the user input interface 605 to input informationinto the data processing system 600, and can include, for example, akeyboard, a mouse, a touch screen, and/or optical or wireless computerinput devices (not shown). The user can input control instructions viauser input interface 605 to control the operation of the ion mobilityspectrometer 100. For example, the user can input parameters to adjustthe operation of the data processing system 600 and/or the ion mobilityspectrometer.

The mode polarity manager 608 can be configured to manage the variousvoltages associated with components of the ion mobility spectrometer100, such as the repelling grid 125, the fixed grid 135, the gating grid145 (in closed mode, for example), the drift rings 160, and the guardgrid 175. The mode polarity manager 608, can be configured to controlwhen and in what order the various components change polarities as theion mobility spectrometer 100 changes modes.

One or more modules of the data processing system 600 can be used toimplement, for example, a determination of certain characteristics ofplasmagram peaks and whether the characteristics are withinpredetermined and/or derived ranges. Further, one or more modules of thedata processing system 600 disclosed consistent with FIG. 6 can be usedto implement a method for normalizing plasmagram data as describedbelow. Further, the storage 602 can be used, for example, to store datarelating to a detection library (such as in the data library 612), whichcan include characteristics of plasmagram peaks of known materialsand/or other data such as reduced ion mobility values. The storage 612can also be used, for example, to store timing information relating toswitching frequencies or clear-down periods consistent with embodimentsof the present disclosure.

Molecules and atoms that are analyzed by the ion mobility spectrometer100 can, from time to time, generate a large peak in a plasmagram.Following such events, residual sample, such as molecules, atoms, and/orions associated with the creation of that peak can remain in theionization region 140 or elsewhere in the ion mobility spectrometer 100.The process of removing these residual materials can be referred to as“fast clearing down.” A fast clear-down operation can be achieved byrapidly switching the ion mobility spectrometer 100 from positive ionmode to negative ion mode. Exemplary time ranges for operation in onemode (i.e., one of positive ion mode and negative ion mode) beforeswitching to the other mode to expedite clear-down can be less thanapproximately 1 second. For example, in one fast clear-down mode, theion mobility spectrometer 100 can operate in positive ion mode forapproximately 20 milliseconds before switching to negative ion mode. Infast clear-down mode, no sample may be introduced so the ion mobilityspectrometer can remove residual sample introduced before receipt of aclear-down trigger. That is, the ion mobility spectrometer 100 canoperate in positive ion mode consistent with one scan period beforeswitching to negative ion mode. In a further fast-switching clear-downmode, the ion mobility spectrometer 100 can operate in positive ion modefor approximately 1 second before switching to negative ion mode. Underthis clear-down mode, the ion mobility spectrometer 100 can operate inpositive ion mode consistent with 40 scan periods before switching tonegative ion mode.

As described above, a number of circumstances can trigger a fastclear-down. For example, clear down can be triggered when certain ionsof interest such as ions corresponding to explosives or contraband drugsare detected by the collector 170. Such ions can have the capacity topersist in the ionization region 140. If, for example, the ionizationregion 140 is not cleared of these ions, then subsequent readings can becontaminated and yield inaccurate results. In this way, the ion mobilityspectrometer can fast clear-down in order to purge sample residue, e.g.,ions corresponding to the sample, left from previous operation (e.g.,run) that may remain in the ionization region 140.

In one embodiment, the ion mobility spectrometer 100 in fast-switchingclear-down mode can acquire current information from the collector 170during the fast clear-down operation to determine if a detectedplasmagram amplitude associated with a residual ion persists. If theacquired current information indicates that a plasmagram amplitudeassociated with a residual ion persists, then that information can be anindication that further fast-switching clear-down operation may bewarranted. In an additional embodiment, the ion mobility spectrometer100 in fast-switching clear-down mode can operate in fast clear-downmode for a time period that can be predetermined based upon detected orpreset criteria. For example, the ion mobility spectrometer 100 can beconfigured to operate in fast clear-down mode for approximately twominutes. The predetermined time period can be based upon the particularstate of the ion mobility spectrometer 100, or can be based upon aclear-down trigger as described herein.

In another embodiment, clear down can be triggered when the ion mobilityspectrometer 100 is being powered on or powered off. Clearing down theionization region 140 and drift region 165 just prior to and/or after aperiod of non-use can assist in the maintenance of the ion mobilityspectrometer 100.

In another embodiment, fast clear down can be triggered periodicallyduring operation of the ion mobility spectrometer 100. For example, thefast clear down operation can trigger automatically every hour, or moreor less frequently, depending on preference.

In another embodiment, clear down can be triggered when the ion mobilityspectrometer 100 detects either the presence or absence of particularplasmagram peaks. The presence of plasmagram peaks corresponding tocontaminants such as sorbitols, nitrates, or fingerprint oils cantrigger fast clear-down. The absence of peaks corresponding to a knowndopant or calibrant, for example, could trigger clear down because theirabsence can be an indication that the ion mobility spectrometer 100 isnot operating in accordance with specifications. In positive ion mode,water is a substance may be present in the ionization region 140. Innegative ion mode, oxygen is a substance may be present in theionization region 140. Therefore, the absence of either of thesesubstances during a test under the respective mode could trigger a cleardown operation.

Fast Clear down operation can be triggered by the detection by the ionmobility spectrometer 100 and associated data processing systemincluding software of any plasmagram peak which exceeds predeterminedranges for characteristics such as amplitude or intensity.

In one embodiment, illustrated in FIG. 7, the ion mobility spectrometer100 can be operated in fast clear-down mode upon acquiring a signal thatsatisfies certain parameters, such as exceeding a certain size,occurring at a certain position, and/or exceeding a certain width. Sucha signal can indicate the presence of a residual molecule, atom, and/orion in the ion mobility spectrometer 100.

Step 704 corresponds to the acquisition of scan data by the ion mobilityspectrometer 100. Data corresponding to a single scan can be acquired byoperation of the ion mobility spectrometer in either positive ion modeor negative ion mode as described above. In addition, as has beendiscussed earlier, an exemplary time period for such a single scan canbe 25 milliseconds. A set of scan data can correspond to a plurality ofsuch scans. Data corresponding to the scan data acquired in step 704 canbe conveyed to the processor 601 through the collector interface 603.

In step 706, segment data is generated from the set of scan dataacquired in step 704. For example, a plurality of scans can be co-addedto form a single segment. The operation associated with step 706 canreduce the signal-to-noise associated with the acquisition of scan dataand can be performed by the processor 601 in accordance withinstructions loaded into the memory module 604 from the storage 602.

In step 708, the segment data generated in step 706 can be processed bythe processor 601 to identify any characteristics of the segment datathat may correspond to a clear-down trigger. This operation can beperformed by the processor 601 in accordance with signal processinginstructions loaded into the memory module 604 from the storage 602.Without limitation, characteristics of the segment data generated by theion mobility spectrometer 100 that can serve as the basis for triggeringa clear-down operation. Examples include, but are not limited to anamplitude of a peak in the segment data at a particular drift time inpositive ion mode or negative ion mode, a full-width-half-maximum of apeak in the segment data at a particular drift time (in positive ionmode or negative ion mode), and an integral of a peak in the segmentdata at a particular drift time (again in positive ion mode or negativeion mode).

In step 710, processor 601 can determine whether any of the clear-downcharacteristics identified in step 708 satisfy criteria or conditions togenerate a clear-down trigger. The data library 612 can contain acollection of information relating to such criteria or conditions. Forexample, the data library 612 can include a data collection, stored in alookup table or some other tabular form, a plurality of drift timescross-referenced to peak amplitudes, peak FWHM, peak integrals, positiveion mode or negative ion mode, reduced ion mobility values, etc.

For example, ions associated with explosives or narcotics can persist inthe ion mobility spectrometer 100 after they have been introduced foranalysis, and can thereby generate both a large initial signal and aresidual signal that can interfere with subsequent analysis by the ionmobility spectrometer 100. Accordingly, at step 710, processor 601 candetermine whether the generated segment data according to one of apositive ion mode or a negative ion mode exhibits a peak at a drift timecorresponding to an ion associated with a target substance, such as TNTor cocaine. If the segment data indicates the presence of such a peak inthe appropriate ion mode, and the peak has a relative amplitude thatexceeds a specified detection threshold (e.g., three times the detectionthreshold at that position) processor 601 can generate a clear-downtrigger. Consistent with the current disclosure, other criteriasufficient to generate a clear-down trigger can include a peak in theappropriate ion mode where the FWHM of the peak exceeds a specifiedthreshold (e.g., 1.5 times an expected peak at that drift time), and/ora peak in the appropriate ion mode where the integral of the peak over adrift time exceeds a specified threshold that, for example, may be threetimes that of the detection threshold for a peak at that drift timeposition. Other criteria that might serve as a clear-down triggerinclude the lack of detection of an expected peak, such as a peakassociated with a dopant, a calibrant, water, or oxygen, or a particularresult of a health check operation.

If processor 601 determines that the characteristics associated with thesegment data do not satisfy the clear-down criteria, then the ionmobility spectrometer 100 can continue operation without clear-down(indicated by step 716). Alternatively, if processor 601 determines thatthe characteristics associated with the segment data do satisfy theclear-down criteria, then a clear-down trigger can be generated and ionmobility spectrometer 100 can implement an option for fast clear-down,which is reflected in step 712. For example, ion mobility spectrometer100 can be configured to provide an indication to a user, through thedisplay 606, that a clear-down operation may be warranted. The ionmobility spectrometer 100 can provide as an option to the user theability to select the clear-down option and implement the fast-switchingclear-down. For example, upon notification to the user through thedisplay 606, the user interface 605 can be configured to accept aselection by the user to either implement the fast clear-down process,or to not implement the fast-switching clear-down process. Uponselection of the fast-switching clear-down option by the user, the ionmobility spectrometer 100 can be configured to implement fastclear-down.

In one embodiment, fast clear-down can be managed by the processor 601and the mode polarity manager 608, where the gating grid 145 ismaintained in a closed configuration (as described further below), and aset number of changes from one polarity mode to the other polarity modeare implemented at a set frequency. Alternatively, and withoutlimitation, fast clear-down can be accomplished by maintaining thegating grid 145 in a closed configuration, and by triggering changesfrom one polarity mode to another polarity mode at a set frequency for aset period of time—again, which can be implemented by the processor 601and mode polarity manager 608.

For example, the processor 601 and mode polarity manager 608 can managea fast clear-down mode by generating instructions and controlling thevoltages on the ion mobility spectrometer 100 such that the ion mobilityspectrometer 100 alternates between positive ion mode to negative ionmode at a period of about 25 milliseconds and continues such alternatingfor about two minutes. The switching can take approximately 2milliseconds. The time period after switching and before the voltagesreach equilibrium can be less than about 5 milliseconds. Where aseparate processor manages the fast-clear-down process, processor 601can generate a clear-down trigger for such processor.

In a further embodiment consistent with the current disclosure, anddepicted in FIG. 8, processor 601 may tailor fast clear-downinstructions depending upon the particular clear-down characteristicsthat are present in the segment data. Again, as has been described inconnection with FIG. 7, step 804 corresponds to the acquisition of scandata by the ion mobility spectrometer 100. Data corresponding to asingle scan can be acquired by operation of the ion mobilityspectrometer in either positive ion mode or negative ion mode asdescribed above, e.g., 25 milliseconds. A set of scan data cancorrespond to a plurality of such scans. Data corresponding to the scandata acquired in step 804 may be conveyed to the processor 601 throughthe collector interface 603.

In step 806, segment data is generated from the set of scan dataacquired in step 804. For example, a plurality of scans can be co-addedto form a single segment. The operation associated with step 806 canreduce the signal-to-noise associated with the acquisition of scan dataand can be performed by the processor 601 in accordance withinstructions loaded into the memory module 604 from the storage 602.

In step 808, the segment data generated in step 806 can be processed toidentify any characteristics of the segment data that may correspond toa clear-down trigger. Without limitation, characteristics of the segmentdata generated by the ion mobility spectrometer 100 that can result in atrigger for a clear-down operation can be the amplitude of a peak in thesegment data at a particular drift time in positive ion mode or negativeion mode, the full-width-half-maximum of a peak in the segment data at aparticular drift time (in positive ion mode or negative ion mode), andthe integral of a peak in the segment data at a particular drift time(again in positive ion mode or negative ion mode).

In step 810, processor 601 can determine whether any of the clear-downcharacteristics identified in step 808 satisfy criteria or conditions togenerate a clear-down trigger. The data library 612 can contain acollection of information relating to such criteria or conditions. Forexample, the data library 612 may include a data collection, stored in alookup table or some other tabular form, a plurality of drift timescross-referenced to peak amplitudes, peak FWHM, peak integrals, positiveion mode or negative ion mode, reduced ion mobility values, etc.

For example, and as has been discussed above, ions associated withexplosives or narcotics can persist in the ion mobility spectrometer 100after they have been introduced for analysis, and can thereby generateboth a large initial signal and a residual signal that can interferewith subsequent analysis by the ion mobility spectrometer 100.Accordingly, at step 810, processor 601 can determine whether thegenerated segment data according to one of a positive ion mode or anegative ion mode exhibits a peak at a drift time corresponding to anion associated with a target substance, such as TNT or cocaine. If thesegment data indicates the presence of such a peak in the appropriateion mode, and the peak has a relative amplitude that exceeds a specifiedthreshold. Other criteria sufficient to generate a clear-down triggercan include, but are not limited to, a peak in the appropriate ion modewhere the FWHM of the peak exceeds a specified threshold and/or a peakin the appropriate ion mode where the integral of the peak over a drifttime exceeds a specified threshold that, for example, may be defined asthree times the detection threshold for a peak at that drift timeposition. Other criteria that might serve as a clear-down triggerinclude absence of an expected peak, such as a peak associated with adopant, a calibrant, water, or oxygen; or a particular result of ahealth check operation.

If processor 601 determines that the characteristics associated with thesegment data do not satisfy the clear-down criteria, then the ionmobility spectrometer 100 can continue operation without clear-down(indicated by step 818). Alternatively, if processor 601 determines thatthe characteristics associated with the segment data do satisfy theclear-down criteria, then specific clear-down instructions associatedwith the detected characteristics can be generated and the ion mobilityspectrometer 100 can implement an option for fast-switching clear-downaccording to these instructions. In step 812, processor 601 can generatespecific clear-down instructions.

For example, it can be determined that, where the amplitude of the peakassociated with TNT is determined to be more than three times a storedor derived value, the residual presence of TNT in the ion mobilityspectrometer 100 can be removed by maintaining the gating grid 145 in aclosed state, and by alternating between positive ion mode and negativeion mode at a frequency of 30 Hz for about 2 minutes. Alternatively,where the amplitude of the peak associated with cocaine is determined tobe about three times a stored value it can be determined that theresidual presence of cocaine in the ion mobility spectrometer 100 can beremoved by maintaining the gating grid 145 in a closed state and byalternating between positive ion mode and negative ion mode at afrequency of 10 Hz for about 1 minute. Accordingly, depending upon thevalue of the clear-down characteristics, processor 601 can generatespecialized clear-down instructions. Alternatively, particular sequencesof mode polarity switching (i.e., a frequency of switching and a totalduration) can be stored in storage 602, and the clear-down instructionsgenerated by the processor 601 can comprise the address in storage 602memory associated with the stored sequence. Although the exemplaryfast-switching clear-down sequences described above relate to a singlefrequency (e.g., 30 Hz or 10 Hz) for a particular duration (e.g., 2minutes or 1 minute) such sequences are exemplary only and are notlimiting. It can be determined, for example, that a variation infrequency can be useful for clearing-down ion mobility spectrometer 100consistent with the current disclosure. Again, by way of example onlyand without limitation, a particular fast clear-down sequence caninclude alternating between positive ion mode and negative ion mode at arelatively high frequency (e.g., 40 Hz) and, over a time of about twominutes, transitioning such a high frequency switching to a lowerfrequency switching (e.g., over the course of two minutes, reducing thefrequency of alternating between positive ion mode and negative ion modefrom about 40 Hz to about 1 Hz). It can also be determined that aclear-down sequence should be available that exhibits no regularfrequency. That is, the amount of time spent in positive ion mode beforeswitching to negative ion mode may not exhibit any regularity (from thestandpoint of time periods) over two or more consecutive polarityswitches.

In step 814, processor 601 can implement an option for fast clear-down.For example, ion mobility spectrometer 100 can be configured to providean indication to a user, through the display 606, that a clear-downoperation may be warranted. The ion mobility spectrometer 100 canprovide as an option to the user the ability to select the clear-downoption and implement the fast clear-down. For example, upon notificationto the user through the display 606, the user interface 605 can beconfigured to accept a selection by the user to either implement thefast-clear-down process, or to not implement the fast clear-downprocess. Upon selection of the fast clear-down option by the user, theion mobility spectrometer 100 can be configured to implement fastclear-down.

According to step 816, a processor 601 and mode polarity manager 608 canaccess the stored clear-down sequences as desired. Again, where aseparate processor manages the fast clear-down process, processor 601can generate a clear-down trigger for such a processor, and theprocessor can access stored sequences as desired.

In another embodiment depicted in FIG. 9, scan data and segment data canbe acquired and processed during the fast-switching clear-down modeoperation, and fast-switching clear-down mode can continue until dataassociated with any residual signal (such as a peak amplitude, FWHM,and/or integral of peak area) indicates that the residual signal hasdropped below a threshold. Such thresholds can vary based on the ionwhich is associated with the residual signal. Similar to the peakcharacteristics discussed above, such thresholds can be in the datalibrary 612, which can include data relating to potential detected ionsand their corresponding drift times, along with corresponding clear-downthreshold values.

In FIG. 9, step 904 corresponds to the acquisition of scan data by theion mobility spectrometer 100. Again, data corresponding to a singlescan can be acquired by operation of the ion mobility spectrometer ineither positive ion mode or negative ion mode as described above. Inaddition, as has been discussed earlier, an exemplary time period forsuch a single scan can be 25 milliseconds. A set of scan data cancorrespond to a plurality of such scans. Data corresponding to the scandata acquired in step 904 may be conveyed to the processor 601 throughthe collector interface 603.

In step 906, segment data is generated from the set of scan dataacquired in step 904. For example, a plurality of scans can be co-addedto form a single segment. The operation associated with step 906 canreduce the signal-to-noise associated with the acquisition of scan dataand can be performed by the processor 601 in accordance withinstructions loaded into the memory module 604 from the storage 602.

In step 908, the segment data generated in step 906 can be processed bythe processor 601 to identify any characteristics of the segment datathat may correspond to a clear-down trigger. Again, this operation canbe performed by the processor 601 in accordance with signal processinginstructions loaded into the memory module 604 from the storage 602.Without limitation, characteristics of the segment data generated by theion mobility spectrometer 100 that can result in a trigger for aclear-down operation can be the amplitude of a peak in the segment dataat a particular drift time in positive ion mode or negative ion mode,the full-width-half-maximum of a peak in the segment data at aparticular drift time (in positive ion mode or negative ion mode), andthe integral of a peak in the segment data at a particular drift time(again in positive ion mode or negative ion mode).

In step 910, processor 601 can determine whether any of the clear-downcharacteristics identified in step 908 satisfy criteria or conditions togenerate a clear-down trigger. The data library 612 can contain acollection of information relating to such criteria or conditions. Forexample, the data library 612 may include a data collection, stored in alookup table or some other tabular form, a plurality of drift timescross-referenced to peak amplitudes, peak FWHM, peak integrals, positiveion mode or negative ion mode, reduced ion mobility values, etc.

For example, ions associated with explosives or narcotics can persist inthe ion mobility spectrometer 100 after they have been introduced foranalysis, and can thereby generate both a large initial signal and aresidual signal that can interfere with subsequent analysis by the ionmobility spectrometer 100. Accordingly, at step 910, processor 601 candetermine whether the generated segment data according to one of apositive ion mode or a negative ion mode exhibits a peak at a drift timecorresponding to an ion associated with a target substance, such as TNTor cocaine. If the segment data indicates the presence of such a peak inthe appropriate ion mode, and the peak has a relative amplitude thatexceeds a specified threshold that, for example, may be defined asapproximately three times that of the detection threshold for a peak atthat position, processor 601 can generate a clear-down trigger. Othercriteria sufficient to generate a clear-down trigger can include a peakin the appropriate ion mode where the FWHM of the peak exceeds aspecified threshold that, for example, may be defined as 1.5 times thatof an expected peak at that drift time position, and/or a peak in theappropriate ion mode where the integral of the peak over a drift timeexceeds a specified threshold that, for example, may be defined as threetimes that of the detection threshold for a peak at that drift timeposition. Other criteria that might serve as a clear-down triggerinclude the lack of detection of an expected peak, such as a peakassociated with a dopant, a calibrant, water, or oxygen; or a particularresult of a health check operation.

If the processor 601 determines that the characteristics associated withthe segment data do not satisfy the clear-down criteria, then the ionmobility spectrometer 100 can continue operation without clear-down(indicated by step 924). Alternatively, if processor 601 determines thatthe characteristics associated with the segment data do satisfy theclear-down criteria, then specific clear-down instructions associatedwith the detected characteristics can be generated and the ion mobilityspectrometer 100 can implement an option for fast clear-down accordingto these instructions.

In step 912, processor 601 can implement an option for fast-switchingclear-down. For example, ion mobility spectrometer 100 can be configuredto provide an indication to a user, through the display 606, that aclear-down operation may be warranted. The ion mobility spectrometer 100can provide as an option to the user the ability to select theclear-down option and implement the fast clear-down. For example, theuser interface 605 can be configured to accept a selection by the userto either implement the fast-switching clear-down process, or to notimplement the fast clear-down process. Upon selection of the fastclear-down option by the user (step 915), the ion mobility spectrometer100 can be configured to implement fast clear-down.

According to step 914, the ion mobility spectrometer 100 can acquirescan data during fast-switching clear-down. Accordingly, voltage pulsescan be sent the gating grid 145 to open the gating grid 145 at scanperiod intervals. In addition, scan data can be acquired from thecollector 170. Upon the acquisition of one or more sets of scan data, instep 916 the processor 601 can generate segment data from the scan dataacquired in step 914. Note that if there is only one set of scan dataacquired in step 914, the segment data generated in step 916 and thescan data acquired in step 914 can be identical. In step 918, thesegment data can be processed by the processor 601 to determine whetherthe segment data includes any clear-down characteristics. At step 920,processor 601 can compare the clear-down characteristics identified instep 918 and determine whether there is any remaining residual signal.If there is no remaining residual signal, then fast-switching clear-downmode can conclude, as indicated by step 924. Otherwise, the ion mobilityspectrometer 100 can continue in fast clear-down mode (step 922). Theloop represented by steps 922, 914, 916, 918, and 920 can continue untilany residual signal acquired during the fast clear-down process dropsbelow a threshold. Again, where a separate processor manages thefast-switching clear-down process, processor 601 can generate aclear-down trigger for such a processor, and the processor can managethe fast clear-down process as described. The fast clear-down processcan have some effect on the values measured by the collector 170. Theseeffects are described below, as are systems and methods that can be usedto address these effects.

In another embodiment, illustrated in FIG. 10, the ion mobilityspectrometer 100 can be configured to implement a fast clear-downprocess according to a maintenance state of the ion mobilityspectrometer 100.

In step 1004, processor 601 can determine the maintenance state of theion mobility spectrometer 100. For example, the ion mobilityspectrometer 100 can be configured to initiate a fast clear-down processupon powering up or powering down. Processor 601 can be configured toidentify such a state (step 1004) and determine whether the maintenancestate satisfies clear-down criteria (step 1006). If not, the ionmobility spectrometer 100 can continue operation (or power down) withoutimplementing a fast-switching clear-down.

In the event that the maintenance state of the ion mobility spectrometersatisfies the clear-down criteria, then processor 601 can generateclear-down instructions (step 1008). The sequence of steps 1008, 1010,and 1012 can be similar to the sequence of steps described in connectionwith FIG. 8 (steps 812, 814, and 816). However, rather than generatingspecialized clear-down instructions associated with a particular signalacquired through scan data (e.g., step 812), the clear-down instructionsgenerated in step 1008 and accessed in step 1012 can be specializedaccording to the maintenance state of the ion mobility spectrometer 100.For example, as described above, a power-up state or a power-down statecan be associated with a particular fast clear-down sequence.Furthermore, periodic operation can also be associated with a particularfast clear-down sequence, such as providing an option to a user everyhour to select a fast clear-down mode, or an option to perform thefast-switching clear-down sequence after every sample performed or afterevery threat detection, or alarm reported.

Fast Switching Operation

FIG. 11 is a plot of exemplary voltages on the repelling grid 125(dashed curve 1130), the gating grid 145 (solid curve 1120), and thefixed grid 135 (dotted curve 1110), such as the ion mobilityspectrometer 100 operating during a transition from positive ion mode(Region I 1140) to negative ion mode (Region III 1160). In Region I 1140(positive ion mode), the voltage on the repelling grid 125 (the dashedcurve 1130) is more positive than the voltage on the gating grid 145(the solid curve 1120), which is more positive than the voltage on thefixed grid 135 (dotted curve 1110). These relative voltage magnitudescan correspond to a gating grid 145 that is closed in positive ion mode.In Region III 1160 (negative ion mode), the voltage on the repellinggrid 125 (dashed curve 1130) is more negative than the voltage on thegating grid 145 (solid curve 1120), which is more negative than thevoltage on the fixed grid 135 (dotted curve 1110). These relativevoltage magnitudes can correspond to a gating grid 145 that is closed innegative ion mode. Between the two regions, Region I 1140 and Region III1160, and before dotted curve 1110 (which corresponds to the voltage onthe fixed grid 135) crosses solid curve 1120 (which corresponds to thevoltage on the gating grid 145), dashed curve 1130 (which corresponds tothe voltage on the repelling grid 125) is kept at a higher potentialthan both solid curve 1120 and dotted curve 1110—indicating that thevoltage on the repelling grid 125 will continue to be more positive thanthe voltage on both of the gating grid 145 and the fixed grid 135.Accordingly, during polarity switchover in Region II 1150, when therecan be negative ions present in the ionization region 140, and where therelative voltages between the gating grid 145 and the fixed grid 135correspond to an open gate in negative ion mode, the repelling grid 125(the dashed curve 1130) is kept high relative to the gating grid 145(solid curve 1120). The relative voltage depicted in FIG. 11 between therepelling grid 125 and the gating grid 145 can keep the negative ionsaway from the gating grid 145 (and thereby the drift region 165) untilafter the dotted curve 1110 crosses the solid curve 1120. When dottedcurve 1110 crosses solid curve 1120 and the relative voltage of thegating grid 145 is less than the voltage of the fixed grid 135, thegating grid 145 is closed in negative ion mode. After that occurs, andthe gating grid 145 is closed, the magnitude of the voltage on therepelling grid 125 can pass below both the voltage of the gating grid145 and the voltage of the fixed grid 135, thereby repelling thenegative ions in the ionization region 140 towards the fixed grid 135and the gating grid 145. In an embodiment, the time that the ionmobility spectrometer 100 spends in Region II 1150 can be approximately2 milliseconds.

Although it is not depicted, a similar sequence of inverted crossingscan be used consistent with the current disclosure to pass fromoperation in negative ion mode with a closed gating grid 145 tooperation in positive ion mode with a closed gating grid 145. Again, thevoltage on the repelling grid 125 can be kept low relative to thevoltage of both the gating grid 145 and the fixed grid 135 until thevoltage of the gating grid 145 crosses over and becomes greater than thevoltage on the fixed grid 135. At that point, as has been described, thegating grid 145 has become closed in positive ion mode, and then therepelling grid 125 can cross both the voltage of the fixed grid 135 andthe voltage of the gating grid 145 and create the potential gradient inthe ionization region 140 that drives the positive ions towards thegating grid 145. FIG. 12 depicts a portion of a cross section of the ionmobility spectrometer 100 consistent with another embodiment. Theportion of the ion mobility spectrometer 100 depicted in FIG. 12includes the collector 170, the guard grid 175, a guard clamping ring1210, an insulator 1290, an insulator 1220, a grounded mount 1295, and aground shield 1240. In the embodiment depicted in FIG. 12, the insulator1290 and the insulator 1220 can comprise ceramic material. In FIG. 12,the grounded mount 1295 can be on the opposite side of the insulator1290 and the insulator 1220 from the collector 170. In an embodiment,the insulator 1290 and the insulator 1220 can be two washers. There canalso be an insulator between the grounded mount 1295 and theguard-clamping ring 1210. This insulating material can be a thin filmsuch as, for example, KAPTON, a polyimide film developed by DuPont.Thus, in an embodiment, there is no direct insulation (other than air)between the guard grid 175, the guard-clamping ring 1210, and thecollector 170, which can reduce the effect that dielectric absorptioncan have on contributing to a current detected at the collector 170during polarity switchover.

In addition, the drift region 165 can be a source of current with anoutput current of around 10-100 pA. Accordingly, there can be aparasitic capacitance between the collector 170 and the guard grid 175of approximately 1 pF where the voltage between these two components canbe around 90 Volts. This can result in an accumulated charge on thecollector 170 (and the guard grid 175) of about 100 pC.

During a rapid polarity switch, the accumulated charge can reverse sign,such that during rapid polarity switching involving many polarityswitches, the accumulated charge can be reversed many times, where thepeak current (i.e., (the change in charge)/(the change in time)) can beapproximately ˜100 pC/1 ms˜100 nA, and which can be 1,000 times largerthan the typical output current from drift region 165. After a polarityswitches, the voltages can stabilize in approximately 1-2 millisecondsor longer consistent with an embodiment.

A preamplifier associated with an ion mobility spectrometer can be atransimpedance amplifier that uses a high input impedance operationalamplifier along with a relatively large feedback impedance (˜GΩ) and canbe incapable of handling input currents much greater than a few hundredpA. To allow for the preamplifier associated with FIG. 2 to handle 100nA currents associated with polarity switchovers, a parallel circuit canbe implemented as indicated.

Where such a parallel circuit includes diodes connected between theinput and the ground that limit the input voltage to safe value, therecan remain some charge stored on the diode capacitance (and otherparasitic capacitances) following a switching current that can take arelatively long time to relax and cause a distortion in the baseline ofthe output signal (i.e. a distortion in the plasmagram).

FIG. 13 shows an embodiment of the preamplifier. Circuit 1300 includes afirst stage 1320 (i.e., a transimpedance integrator circuit), which caninclude a first operational amplifier 1301, a feedback capacitor C1 1302and a switch S1 1303. The feedback capacitor C1 1302 can be, forexample, 2 pF. In an embodiment, the first operational amplifier 1301,the feedback capacitor C1 1302 and the switch S1 1303 can be availableas a single integrated circuit 1304. The integrated circuit (IC) 1304can be, for example, IVC102 manufactured by Texas Instruments. The IC1304 can be chosen to meet a user's specifications for the switchleakage current and charge injection. Where the IC 1304 is IVC102 asdescribed above, the manufacturer's specified input current (includesoperational amplifier bias current and switch leakage) can be 0.1 pA.The charge injection can be as small as 0.2 pC. In a further embodiment,the leakage current can be much smaller than the magnitude of a signalacquired from the collector 170 (<1 pA). Because a semiconductor switchcan inject a charge during opening and closing operation, the amount ofcharge injection associated with the low-leakage switch 1303 in anembodiment consistent with the disclosure can be selected to be below 1pC. A second stage 1305 (i.e., a differentiator circuit) can be basedaround an operational amplifier 1306 which can be, for example, a lownoise, precision operational amplifier such as OP27 manufactured byAnalog Devices and includes a resistor R1 1307 and a capacitor C2 1308.The resistor R1 1307 can be, for example, 100 kΩ, and the capacitor C21308 can be, for example, 22 nF. The total transimpedance (ratio ofoutput voltage to input current) of the circuit 1300 can be given byR1·C2/C1 and may be, for example, 1 GΩ. Other values for the resistor R11307, the capacitor C1 1302, and the capacitor C2 1308 can include, forexample, about 300 kΩ for the resistor R1 1307, about 10 pF for thecapacitor C1 1302, and 33 nF for the capacitor C2 1308. Generally, thevalues for the resistor R1 1307, the capacitor C1 1302, and thecapacitor C2 1308 can depend upon the application (e.g., the desiredgain).

The switch 1303 can be closed just before the start of the polaritytransition, and can remain closed during fast-polarity switchover beforeopening a few milliseconds later when all grid voltages (such as thegrid guard 175 voltage) have stabilized. Generally, the time periodbetween the closing and the subsequent opening of the switch 1303 can beless than 5 ms. The timing of the switch 1303 can be digitallycontrolled by the RESET logic signal 1310, which can be generated by theprocessor 601. One aspect of the circuit 1300 consistent with thepresent disclosure is that transients introduced by the circuit 1300 canbe small in magnitude, thereby avoiding contributions to distortions inthe baseline of the output signal.

FIG. 14 shows timing of the Trace 1401 shows schematically the polarityof the ion mobility spectrometer 100 transitioning from positive ionmode 1411 to negative ion mode 1412 and back to positive ion mode 1413.

Trace 1402 shows the timing of a RESET signal pulse 1421, associatedwith RESET logic signal 1310 of switch 1303, and which can be digitallycontrolled by the processor 601. This can be a logic signal active LOW.It can be asserted just before the start of the ion mobilityspectrometer 100 polarity transition from positive ion mode 1411 tonegative ion mode 1412 and can end after all the ion mobilityspectrometer 100 voltages have stabilized. Another RESET signal pulse1422 can be asserted just before the start of the ion mobilityspectrometer 100 polarity transition from negative ion mode 1412 topositive ion mode 1413. The RESET signal pulses 1421 and 1422 can lastabout 2 milliseconds.

Trace 1403 depicts a GATING pulse signal 1431 that can mark thebeginning of the plasmagram data collection in negative ion mode 1412,and depicts a GATING pulse signal 1432 that can mark the beginning ofthe plasmagram data collection in positive ion mode 1413 (that is,GATING pulse signal 1432 can mark the beginning of the collection ofdata associated with a scan). The GATING pulses 1431 and 1432 can beconfigured to occur about 10 milliseconds after the ion mobilityspectrometer 100 voltages have stabilized in either negative ion mode1412 or positive ion mode 1413. This can allow for the ions within thedrift region 165 of the ion mobility spectrometer 100 to establish a newequilibrium corresponding to the polarity thereby stabilizing thebaseline current of the collector 170 of the ion mobility spectrometer100.

Trace 1404 is a preamplifier output. When the RESET signal pulse 1421 isasserted, the trace 1404 can show a spike 1441 due to the discharging ofcapacitor C1 1302. Then, for the duration of the RESET signal pulse1421, the output can be essentially 0 volts. At the end of the RESETsignal pulse 1421, there can be a small spike 1442 due to chargeinjection. Then there can be a period of a few milliseconds when the ionmobility spectrometer 100 baseline current stabilizes.

Plasmagram data can be collected in scans lasting 20 to 25 millisecondsfollowing the GATING signal pulse 1431. Depending on the implementation,the ion mobility spectrometer 100 can be operated so as to changepolarity after a scan (such as oscillating between positive ion mode1411 and negative ion mode 1412 as shown in trace 1401), or can collectseveral scans in one polarity before switching to the other polarity.For example, a switch of the polarity of the ion mobility spectrometer100 can occur at any number of scans (e.g., every scan, every 5 scans,every 10 scans, or more).

When the ion mobility spectrometer 100 polarity is switched everyseveral scans, the RESET pulse signal 1421 may be asserted after eachscan and released before each GATING pulse signal 1431 or every severalscans as long as the IC 1304 does not saturate.

FIGS. 15 and 16 depict another embodiment of the preamplifier of FIG. 13with a switch. FIG. 15 depicts a circuit 1500 which can be used in placeof IC 1304 of FIG. 13. Two states of the circuit 1500 are shown; the‘SWITCH OPEN’ state depicted in FIG. 15 and the ‘SWITCH CLOSED’ statedepicted in FIG. 16. The ‘SWITCH OPEN’ state depicted in FIG. 15corresponds to the open setting of switch 1303 and the ‘SWITCH CLOSED’state depicted in FIG. 16 corresponds to the closed setting of switch1303—and which is connected to the RESET operation discussed above.

A diode D1 1501 and a diode D2 1502 can be connected between an inputnode 1503 and two switches, S1 1504 and S2 1505. During operation inpositive ion mode or negative ion mode, the diodes 1501 and 1502 can beswitched to ground 1506 and 1514. The input node 1503 can be held atvirtual ground by a feedback capacitor C 1507 across an amplifier 1508.The equivalent series resistance of the diodes 1501 and 1502 at 0 V biascan be very high; close to TΩs therefore, there can be virtually nocurrent flowing through those diodes 1501 and 1502 even if there issmall offset voltage (typically ˜1 mV) present on the input node 1503.

During RESET, (i.e., during polarity switchover), both switches, S1 1504and S2 1505 can be flipped, as illustrated in FIG. 16 thus connectingall four diodes 1501, 1502, 1509 and 1510 into a bridge configurationwith all four diodes 1501, 1502, 1509 and 1510 forward biased. Thebridge can act as a feedback resistor whose resistance equals to theequivalent series resistance of the diodes 1501, 1502, 1509 and 1510.The bias current used can be about 8 μA, the equivalent seriesresistance can be of the order of ˜6 kΩ, which can be much less than theoperating impedance of the circuit 1500 in the ‘SWITCH OPEN’ state asillustrated in FIG. 15. This keeps an output voltage 1513 close to 0 Veven if input current reach 100s of μA.

When the RESET signal terminates, the diodes D1 1501 and D2 1502 can beconnected back to the ground 1506 and 1514, as depicted in FIG. 15.There can be a small charge on each diode 1501 and 1502 equal to forwardvoltage times diode capacitance. However, the two diodes 1501 and 1502can have opposite charges, and when they are discharged, only thedifference caused by mismatch of the charges becomes injected into theinput 1503 of the circuit 1500. The total charge injected can be under 1pA

The amplifier 1508 can be, for example, a dual JFET (junction gatefield-effect transistor), such as SST441 manufactured by VishaySiliconix, driving a high precision low noise operational amplifier suchas OPA2227 manufactured by Texas Instruments. Resistors R1 1511 and R21512 can be 1 MΩ each (i.e., they can be matched), diodes D3 1509 and D41510 can be a dual diode such as MMBD3004S manufactured by DiodesIncorporated, and the switches S1 1504 and S2 1505 can be implementedusing a low capacitance, low charge injunction dual SPDT switch such asADG1236 manufactured by Analog Devices.

Diodes D1 1501 and D2 1502 can be selected to have low capacitance andthe largest possible equivalent series resistance. The design can usep-n junction JFET devices including, for example, SST-J212 manufacturedby Vishal Siliconix.

Where data associated with a plasmagram is acquired duringfast-switching clear-down mode, systems and methods consistent with yetanother embodiment of the present disclosure can take into account anon-linearity that can be introduced into plasmagrams as a result of afast-switching operation. Specifically, it is found that fast-switchingcan introduce a background distortion into the scan data that isprocessed to generate a plasmagram. An example of this distortion isillustrated in the plasmagram 1700 depicted in FIG. 17. Specially, theregion 1701 and the region 1702 exhibit a baseline curve that is noteven with the ordinate. A compensation for the non-linearity of thisdistortion can be accomplished by subtracting a value from a fittedcurve from each value of the plasmagram in real-time before theplasmagram data (such as the segment data or the scan data) is analyzedby the processor 601 for clear-down characteristics. This can reduce theeffect of the plasmagram non-linearity and allow the plasmagrambackground to approach the zero-level of the ordinate (i.e., it cannormalize the measured values in time domain). Such an adjustment canassist in determining whether a clear-down characteristic (such as aresidual signal associated with particular ions) is present in thesegment data.

Steps associated with normalizing plasmagram data can generally bedivided into two parts and is illustrated in FIG. 18. In the first part,an offline calculation 1806 can be performed on a collection 1802 ofsegment data associated with fast-polarity switching to develop, amongother things, fitting coefficients (step 1828). In the second part 1808,these fitting coefficients can be used to subtract a portion of theamplitude from scan data values acquired by the collector 170 inreal-time (i.e., as the scan data is collected, or prior to the analysisof the resulting segment data by the processor 601 for clear-downcharacteristics).

The offline curve fitting calculation 1806 can include several steps.First, a sufficient number of clean plasmagrams (e.g., collections 1802of segment data) can be collected such that a processor (which can beeither processor 601, or another processor), under control of softwareinstructions or otherwise, can perform a least-squares fitting of afitting and calibration area of the plasmagram to a polynomial form(step 1820) or to an exponential form (step 1816). The regions of theplasmagram curves that are fitted—referred to as fitting and calibrationareas—are illustrated in FIG. 17. Specifically, plasmagram 1700 includesFitting and Calibration Area A (no peaks) 1701 and Fitting andCalibration Area B (no peaks) 1702. Fitting and Calibration Areas A 1701and B 1702 can be selected or identified (step 1814) by the lack ofsignificant peaks. Preferably, Fitting and Calibration Areas A 1201 andB 1202 can be selected to exhibit as little noise as possible. Thesegment data 1812 that can be used for the offline calculation 1806 canbe collected—for example—from blank test samples, and when no chemicalshave been introduced to the ion mobility spectrometer 100.

As indicated in FIG. 17, plasmagrams used for purposes of offlinecalculation 1806 can have relatively large fitting areas 1701 and 1702.The processor can perform a least squares fit of the selected fittingarea to an exponential form f_(exp)(t)=ae^(−bt) (step 1816) and define afitting error (step 1818). The processor can also perform a leastsquares fit of the selected fitting area to polynomial form

${f_{poly}(t)} = {{a_{0} + {a_{1}t} + {a_{2}t^{2}} + {a_{3}t^{3}} + \ldots + {a_{N}t^{N}}} = {\sum\limits_{k = 0}^{N}{a_{k}t^{k}}}}$(step 1820) and define a fitting error (step 1822). The regionassociated with the selected fitting areas can be fitted to either apolynomial or an exponential function, whichever gives betterapproximation (step 1824). (i.e., whichever approximation yields asmaller fitting error). The fitting error associated with step 1318 isthe difference between the least squares fit to the exponential formf_(exp)(t)=ae^(−bt) (step 1316) and the plasmagram data. The fittingerror associated with step 1322 is the difference between the leastsquares fit to the polynomial form

${f_{poly}(t)} = {{a_{0} + {a_{1}t} + {a_{2}t^{2}} + {a_{3}t^{3}} + \ldots + {a_{N}t^{N}}} = {\sum\limits_{k = 0}^{N}{a_{k}t^{k}}}}$(step 1320) and the plasmagram data.

Based upon which approximation yields a smaller fitting error (step1824), the processor can determine whether to use the fitting to theexponential curve (step 1816) or the fitting to the polynomial curve(step 1820). This can be based on the value of fitting error, which canitself depend on the level of plasmagram noise. The processor can thenidentify a fitting form and average fitting coefficients (step 1828),which can be the result of many plasmagrams and instruments. Theprocessor can also identify the standard deviation of fitting errorsbetween collected plasmagrams (step 1826). The standard deviation caninclude the change in error associated with the same ion mobilityspectrometer 100 and/or the change in error between different ionmobility spectrometers 100.

The calculation 1808 associated with the second part can be stored assoftware instructions in storage 602 and can be available to processor601 as scan data and/or segment data is made available through collectorinterface 603. The processor 601 can be configured to collect all of theacquired data into a set {r_(n)(t)} (step 1830). For each elementr_(n)(t) of the plasmagram set {r_(n)(t)}, the processor 601 can beconfigured to calculate the corresponding value of the fitted curvedetermined in step 1828 (F(t)) and any associated error (step 1826). Itis possible to use a nested polynomial for faster calculation of F(t).The processor 601 can be configured to subtract this backgroundcontribution F(t) from each element of the plasmagram set {r_(n)(t)}(step 1834). After such subtraction, the non-linear distortion, such asexhibited in the plasmagram 1700, can be eliminated. The end result canbe plasmagram data (step 1810) with a flat baseline. Plasmagram 1704 inFIG. 17 depicts a curve similar to plasmagram 1700, but with thebackground distortion substantially eliminated.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theembodiment disclosed herein. Although one or more methods have beendescribed in conjunction with the ion mobility spectrometer 100 and/orthe data processing system 600, it is to be apparent that the method maybe used with other devices and configurations of ion mobilityspectrometers. It is intended that the specification and examples beconsidered as exemplary only, with a true scope and spirit of theinvention being indicated by the following claims.

We claim:
 1. An ion mobility spectrometer comprising: an ionizationregion and a drift region; wherein the ion mobility spectrometer isconfigured to operate in a clear-down mode to clear-down a residualsample that remains in the ionization region and/or the drift regionwherein the clear-down mode comprises switching between a positive modeand a negative mode a plurality of times, and wherein no sample may beintroduced to the spectrometer during the clear-down mode.
 2. The ionmobility spectrometer of claim 1 wherein the ion mobility spectrometerfurther comprises at least one of a repelling grid, a gating grid, afixed grid and a guard grid, and wherein the clear down comprises atleast one of the repelling grid, the gating grid, the fixed grid and theguard grid switching from positive mode to negative mode, or negativemode to positive mode.
 3. The ion mobility spectrometer of claim 2wherein the switching between positive and negative modes comprises theat least one of the repelling grid, the gating grid, the fixed grid andthe guard grid being switched from a lower voltage to a higher voltageor a higher voltage to a lower voltage relative to at least one other ofthe repelling grid, the gating grid, the fixed grid and the guard grid.4. The ion mobility spectrometer of claim 2 wherein the switchingbetween positive and negative modes comprises at least one of therepelling grid, the gating grid, the fixed grid and the guard grid beingswitched to have an opposite polarity.
 5. The ion mobility spectrometerof claim 2 wherein the at least one of the repelling grid, the gatinggrid, the fixed grid and the guard grid is configured to provide apotential gradient across the ionization region and/or the drift region.6. The ion mobility spectrometer of claim 1 wherein the clear-downcomprises maintaining a gating grid of the ion mobility spectrometer ina closed state.
 7. The ion mobility spectrometer of claim 1 comprising aprocessor configured to issue a clear-down trigger upon detection ofions of the residual sample, wherein the clear-down trigger initiatesthe clear-down mode.
 8. The ion mobility spectrometer of claim 7 whereinthe processor is configured to issue the clear-down trigger upondetection of ions of at least one of a dopant, a calibrant, water, oroxygen during previous operation of the ion mobility spectrometer. 9.The ion mobility spectrometer of claim 7 wherein the processor isconfigured to initiate the clear-down mode in the event that a targetsubstance is detected.
 10. The ion mobility spectrometer of claim 1wherein the clear-down mode is performed: at a selected frequency; at afrequency of at least 1 Hz aperiodically; and at a frequency of between1 Hz and 40 Hz.
 11. The ion mobility spectrometer of claim 10 whereinthe duration of the clear down period is at least one minute.
 12. Theion mobility spectrometer of claim 1 wherein the ion mobilityspectrometer comprises a data processing system that is configured tooutput an option to a user and to accept a selection by the user toimplement a clear-down process.
 13. The ion mobility spectrometer ofclaim 1, wherein the ion mobility spectrometer is configured to operatein the clear-down mode when the ion mobility spectrometer is beingpowered on or powered off.
 14. The ion mobility spectrometer of claim 1wherein the ion mobility spectrometer is configured to operate in theclear-down mode at intervals selected from the list comprising:periodically; after every sample; and after threat detection alarm. 15.The ion mobility spectrometer of claim 1, wherein scan data and segmentdata can be acquired and processed during the fast-switching clear-downmode.
 16. The ion mobility spectrometer of claim 1, wherein thefast-switching clear-down mode is initiated by receipt of a clear-downtrigger.